High voltage switch for ignition systems of internal combustion engines

ABSTRACT

A high voltage switch can be used in an internal combustion engine ignition system to switch a high voltage through to a spark plug upon illumination of a light-sensitive portion of the switch. The high voltage switch includes two sub-stacks of break-over diodes, with the diodes of the two sub-stacks having different properties. The break-over diodes of a first, illuminated sub-stack of the high voltage switch have a break-over voltage with a low dependence on temperature, whereas the break-over diodes of a second, unilluminated sub-stack have a low break-over current with tight tolerances.

FIELD OF THE INVENTION

The present invention relates to a high voltage switch, particularly toa high voltage switch for use in ignition systems of internal combustionengines.

BACKGROUND INFORMATION

German Published Patent Application No. 40 32 131 (the "'131 reference")describes a high voltage switch which assumes the function of anignition voltage distributor with quiescent high voltage distribution.This high voltage switch consists of semiconductor elements which havelight-sensitive zones, and the light-sensitive zones being so actuatedby light-emitting elements that they connect through at a prescribedtime, e.g., in accordance with the firing order. The individualsemiconductor elements in the device of the '131 reference arebreak-over diodes which are arranged in a cascade circuit. Thesebreak-over diodes provide the advantage that when the break-over voltageis reached the high voltage which is built up on the secondary side issuddenly connected through to the spark plugs, so that a voltage losscaused by possible shunt connections, for example due to carbon fouling,is avoided. With the device of the '131 reference, it is not necessaryto irradiate the entire cascade circuit with optical energy. It issufficient to irradiate only part of the break-over diode cascade sothat only some of the break-over diodes are switched by means of lightand the remaining break-over diodes are connected through when theprescribed voltage is reached and as a result a desired gradientsteepening effect is achieved.

SUMMARY OF THE INVENTION

The present invention provides a high voltage switch which has theadvantage that the first illuminated sub-stack of break-over diodes hasa break-over voltage with a low temperature coefficient. However, at thesame time these break-over diodes have break-over currents with a hightemperature coefficient. They are thus particularly suitable for theilluminated part since they do not act here as a break-over diode butrather as a light-triggered switch. As a result of the arrangement ofthe unilluminated part of the high voltage switch, the gradientsteepening effect is maintained. This arises from the overhead ignitionof the illuminated part of the high voltage switch.

It is particularly advantageous that break-over diodes with lowbreak-over currents with tight tolerances are obtained for theunilluminated second sub-stack of the break-over diode cascade since, inthis way, the operation of the break-over diode is maintained for thegradient steepening effect. The combination of these two types ofbreak-over diodes has the advantage that a low temperature coefficientof the switch is obtained and thus the number of semiconductor elementscan be reduced. Finally, it is advantageous that the two sub-stacks ofthe high voltage switch can also consist of two discrete, separatelypacked components. All that has to be ensured here is that theconnecting length between these cascade components be as short aspossible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an ignition system with a highvoltage switch, in accordance with the present invention, having twosub-stacks packed together.

FIG. 2 is a schematic representation of an ignition system with a highvoltage switch, in accordance with the present invention, having twoseparately packed cascade components.

DETAILED DESCRIPTION

FIG. 1 is a diagrammatic view of an internal combustion engine with anignition system and quiescent high voltage distribution, only twoignition coils being illustrated but a multitude of further ignitioncoils being possible. A battery voltage U_(B) is connected to a primarywinding 10 of an ignition coil 11, a switching transistor 12 beingconnected in series with the primary winding 10. The switchingtransistor 12 is continuously actuated by a control device 13 via itsbase terminal in order to produce a secondary-side high voltage to theignition coil 11. A secondary winding 14 of the ignition coil 11 isconnected on the one hand to ground and, on the other hand, to sparkplugs 17 and 18 in each case via a high voltage switch 15 or 16respectively. The high voltage switches 15 and 16 are each divided intotwo sub-stacks 15a and 15b as well as 16a and 16b, the sub-stacks 15aand 16a being arranged on the side assigned to the ignition coil and thesub-stacks 15b and 16b being on the side assigned to the spark plug. Thesub-stacks 15a and 16a are actuated at their light-sensitive regions viaa light emitting source, for example an LED, by the control device 13 inaccordance with the ignition specification and thus transmit theignition voltage to the spark plugs. The transmission of light from thelight-emitting element 19 to the sub-stack 15a or 16a to be illuminatedtakes place by means of a fiber-optic rod 20. However, it is alsoconceivable to have an arrangement of individual sub-stacks in which theunilluminated sub-stack is arranged on the side assigned to the ignitioncoil and the illuminated sub-stack is arranged on the side assigned tothe spark plug.

In FIG. 2, the ignition circuit is constructed exactly as in FIG. 1 sothat identical reference symbols are used for identical parts. However,the high voltage switch has been provided with other reference symbols,since in FIG. 2 the high voltage switch consists of two sub-stacks 21and 22 as well as 23 and 24 which are separated from one another. Withthis separate arrangement of the sub-stacks, all that has to be ensuredis that the connection 25 or 26 between the sub-stacks is to be selectedto be as short as possible in order to keep line capacitances as smallas possible.

Break-over diodes of the unilluminated sub-stack have a relatively lowbreak-over current between 1 mA and approximately 15 mA over the entiretemperature range. These low break-over currents are required for theoperation as a break-over diode with gradient steepening effectBreak-over diodes with relatively low break-over currents have, however,in each case a high temperature coefficient of the break-over voltage,i.e. the break-over voltage decreases as the temperature increases. Ifthe cascade circuit only consisted of the break-over diodes with theseproperties, because of the high temperature coefficient, a high numberof break-over diodes would be required in order to be able to maintainthe desired overall voltage over the entire temperature range.

In contrast with this, the break-over diodes with low temperaturecoefficient of the break-over voltage, such as are used for theilluminated part of the stack, have a high temperature coefficient ofthe break-over currents, so that these break-over diodes are notsuitable for realizing the gradient steepening effect and, instead, canbe used advantageously for the light-triggered part of the break-overdiode cascade.

What is claimed is:
 1. A high voltage switch for coupling between a highvoltage source and a spark plug, the high voltage switch comprising:aplurality of break-over diodes having a preselectable break-over voltagethat can be reduced by irradiation with optical energy, wherein: theplurality of break-over diodes are arranged in a cascade circuit infirst and second sub-stacks, the first sub-stack being selectivelyilluminated with optical energy, and the break-over diodes of the firstsub-stack have a break-over voltage with a lower temperature coefficientthan the break-over diodes of the second sub-stack.
 2. The high voltageswitch of claim 1, wherein the break-over diodes of the second sub-stackhave a second break-over current lower than a first break-over currentof the break-over diodes of the first sub-stack.
 3. The high voltageswitch of claim 2, wherein the second break-over current of thebreak-over diodes of the second sub-stack is between approximately 1 mAand 15 mA.
 4. The high voltage switch of claim 1, wherein the first andsecond sub-stacks are packaged separately and connected in series. 5.The high voltage switch of claim 4, wherein a length of a connectionbetween the first and second sub-stacks is minimized.